1. Field of the Invention
The present invention relates to a fuel cell stack of power generation cells formed by stacking a plurality of electrolyte electrode assemblies and separators alternately. Each of the electrolyte electrode assemblies includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. A fuel gas is supplied to the anode, and an oxygen-containing gas is supplied to the cathode for generating electricity in the power generation cells.
2. Description of the Related Art
For example, a phosphoric acid fuel cell (PAFC) is a power generation cell which employs a porous electrolyte layer of silicon carbide matrix for retaining concentrated phosphoric acid. The electrolyte layer is interposed between carbon-based electrodes (anode and cathode) to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). The electrolyte electrode assembly and the separators make up a unit of the power generation cell for generating electricity. A predetermined number of the power generation cells are stacked together to form the fuel cell stack.
Another type of the power generation cell is a solid polymer electrolyte fuel cell which employs a membrane electrode assembly (MEA) including a polymer ion exchange membrane (proton exchange membrane). Similarly, the membrane electrode assembly and the separators make up a unit of the power generation cell. A predetermined number of the power generation cells are stacked together to form the fuel cell stack.
In the fuel cell stacks, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
The fuel cell should be operated at around an optimum temperature for the performance of power generation. For example, the phosphoric acid fuel cell is operated in the temperature range of 120° C. to 200° C., and the solid polymer electrolyte fuel cell is operated in the temperature range of 60° C. to 90° C. In order to maintain the temperature of the power generation cells in the desirable temperature range, various cooling systems have been adopted. Typically, the power generation cells are cooled by supplying coolant such as water to a coolant passage formed in the separators of the fuel cells stack.
In the cooling system, coolant such as water or conventional cooling liquid for vehicles contains impurities such as ion, or metallic additives. Therefore, the coolant itself is conductive. Even if deionized water or pure water is used as the coolant, the coolant may be contaminated by metals, for example, and become conductive in circulating a coolant passage or a radiator during the operation of the cooling system.
While electrons produced by the electrochemical reactions in the power generation cells are collected from terminal plates at opposite ends of the fuel cell stack, the electrons may be leaked to the coolant passage or the radiator through the conductive coolant undesirably. Consequently, the power generation performance of the overall fuel cell stack is degraded due to energy losses caused by the leakages of electric current to the earth or liquid.
A solution to the problem proposed by the applicant of the present invention is discussed in the U.S. patent application publication No. U.S. 2001/0046618 A1. The publication discloses a fuel cell stack which effectively prevents the electric leakages through the coolant with a simple structure for maintaining a desirable power generation performance.
In the fuel cell stack, cooling cells are interposed between terminal plates for collecting electricity. Insulating means electrically insulates the coolant supplied into the cooling cell from the power generation cells and the terminal plates. Further, conducting means is used for electrical connection between the power generation cells, and electrical connection between the power generation cells and the terminal plates. With this structure, energy losses caused by electricity leakages to the earth or liquid is reliably prevented, and the desirable power generation performance of the overall fuel cell-stack is maintained.
In operating the fuel cell stack, heat is generated. The amount of heat depends on the operating condition of fuel cell stack. When the fuel cell stack is operated at high load, a large amount of heat is generated. In order to radiate heat generated in the operation of the fuel cell stack at the maximum load, the cooling system includes a relatively large heat exchanger.
For example, it is assumed that a fuel cell stack formed by stacking power generation cells are operated at a rated output of 70 kW. The power generation cells have a current density of 1 A/cm2. The voltage applied between terminals of each power generation cell is approximately 0.6V. Under the condition, approximately 12% of the heat generated in the operation of the fuel cell stack is used to keep the temperature of the fuel cell stack, and used by radiation from the fuel cell stack. Therefore, the remaining approximately 88% of the heat needs to be absorbed by the coolant supplied into the fuel cell stack, or radiated from the externally positioned heat exchanger.
Consequently, a large pump is needed for circulating the coolant in the fuel cell stack, or the heat exchanger needs to be considerably large for radiating a large amount of heat.